Abstract
The initiation efficiency and the excited state dynamics of the three triplet radical photoinitiators MMMP (2-methyl-4′-(methylthio)-2-morpholinopropiophenone), benzoin (2-hydroxy-1,2-diphenylethanone, Bz), and 4-methyl benzoin (2-hydroxy-2-phenyl-1-(p-tolyl) ethanone, 4MB) are investigated via a trifold combination of pulsed laser polymerization and subsequent electrospray ionization mass spectrometry (PLP-ESI-MS), femtosecond transient absorption (TA) spectroscopy, and density functional theory (DFT) methods. A quantitative elucidation of the underlying mechanisms that cause unequal initiation efficiencies of the three photoinitiators is proposed suggesting higher radical formation capabilities for MMMP and 4MB compared to Bz based on the inherent photophysical properties of the three initiators. MMMP shows significantly higher complexity of the relaxation pathways due to partial excitation into higher singlet states as well as extended triplet lifetimes. However, Bz shows the highest initiation efficiency for polymerization of MMA at a wavelength of 351 nm compared to both MMMP (Bz : MMMP corresponds to 1 : 0.63) and 4MB (Bz : 4MB corresponds to 1 : 0.86). The current study thus evidences that the combination of PLP-ESI-MS and TA experiments allows for arriving at quantitative initiation abilities of identical radical fragments originating from disparate source molecules. However, the quantitative initiation evaluation of different fragments originating from disparate source molecules requires additional information regarding the fragments' reactivity towards vinyl bonds.
Highlights
The initiation process of photo-induced polymerization reactions is of signi cant importance for the chemical nature of the nal polymer, and for the rate at which polymerization is initiated
The benzoyl fragment B of Bz shows the highest initiation efficiency compared to the radical fragment MB from 4-methyl benzoin (4MB) and the two fragments T and N from MMMP
When comparing the same number of excited molecules, the fragments T and N show an equal or lower initiation efficiency compared to the MB fragment, respectively, due to the lower extinction coefficient of 4MB compared to MMMP
Summary
The initiation process of photo-induced polymerization reactions is of signi cant importance for the chemical nature of the nal polymer, and for the rate at which polymerization is initiated. Type I initiators such as acetophenones, benzyl ketals, or benzoin and its derivatives are excited into a triplet state upon irradiation with UV light and subsequently undergo a-cleavage, forming two radical fragments which are – both or only one of them – capable of initiating a polymerization reaction. The initiator 2-methyl-40-(methylthio)-2-morpholinopropiophenone (MMMP) is widely used in industrial applications and has been referred to as a ‘near-ideal’ photoinitiator, forming two radical fragments with equal initiation abilities.[3] a quantitative evaluation of this statement, e.g., via pulsed laser polymerization and subsequent electrospray ionization mass spectrometry (PLP-ESI-MS) has not yet been carried out. In the current study we compare the initiation efficiency of the radical fragments of the type I photoinitiators MMMP, benzoin (Bz), and 4-methyl benzoin (4MB) (Scheme 1) derived from a post-mortem analysis of their polymerization products via PLP-ESI-MS. In order to achieve an exact comparison of the initiation efficiencies of the aforementioned initiators, the already established PLP-ESI-MS technique[4,5,6] was
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